1. Field of the Invention
The invention relates to devices for measuring the mobility of ions, particularly in gases at pressures of a few hectopascal.
2. Description of the Related Art
Isomers of the primary structure (“structural isomers”) and isomers of the secondary or tertiary structure (“conformational isomers”) possess different geometrical shapes but exactly the same mass. Mass spectrometry is therefore unable to detect that they are different. One of the most efficient methods of recognizing and distinguishing such isomers is to separate them by virtue of their ion mobility. A cell for measuring the ion mobility contains an inert gas (such as helium or nitrogen). The ions of the substance under investigation are usually pulled through the stationary gas by means of an electric field. The large number of collisions with the gas molecules leads to a constant drift velocity vd for every ionic species which is proportional to the electric field strength E: vd=M×E. The proportionality factor M is called the “ion mobility”. The ion mobility M is a function of the temperature, gas pressure, type of gas, ionic charge and, in particular, the collision cross-section. Isomeric ions of the same mass but different collision cross-sections possess different ion mobilities. Isomers with the smallest geometry possess the largest mobility M and therefore the largest drift velocity vd through the gas. Protein ions which are unfolded undergo more collisions than tightly folded proteins. Unfolded protein ions therefore arrive at the end of the cell later than folded ions of the same mass.
A variety of information can be obtained from measurements of the ion mobility M. Measurements of the relative ion mobility are frequently used to investigate conformational changes or merely to discover the existence of different isomeric structures in a mixture. Ions with the same mass-to-charge ratio m/z but different conformation can be separated from each other relatively easily. It is even possible to calculate the absolute collision cross-sections from well reproduced measurements with helium as the gas. Specific folding forms can be confirmed in turn from the accurate collision cross-sections.
Knowledge of the mobility of ions has become more and more important in chemical and biological research, and devices for measuring ion mobility have therefore been incorporated in mass spectrometers in order to combine measurements of the mass-to-charge ratio of ions with measurement of collision cross-sections.
For couplings with mass spectrometers, a pressure range of 500 to 2000 pascals has been adopted almost universally for the mobility drift region; the drift region is 40 centimeters up to two meters and more, and electric field strengths of 1000 to 5000 volts per meter are applied. In this pressure range, the drifting ions form almost no complexes with other substances, so the mobilities of the ionic species can be measured without interferences. In long drift regions, the ions also diffuse apart in the radial direction over long distances, and therefore quite large diameters have to be chosen for long drift regions.
The ions are usually introduced into the drift region in the form of temporally short ion pulses, causing them to adopt the shape of spatially small ion clouds, which are pulled through the drift region by the electric field. These ion clouds are subject to diffusion in the gas of the drift region. The diffusion takes place in both the forward and the backward direction, and also transverse to the drift region. The mobility-resolving power Rmob (mobility resolution for short) is predominantly determined by this diffusion broadening of the ion clouds, especially for long drift regions and low electric field strengths; all other influences, such as the space charge, tend to be infinitesimally small. The mobility resolution Rd, which is calculated solely from the diffusion broadening of the mobility signal, is given by the equation:
      R    d    =            1      4        ⁢                            zeEL          d                          kT          ⁢                                          ⁢          ln          ⁢                                          ⁢          2                    where z is the number of elementary charges e, E the electric field strength, Ld the length of the drift region, k the Boltzmann constant and T the temperature. It can be seen that the diffusion-limited resolution increases with the field strength E, and particularly with the length Ld of the drift region also, albeit only as the square root in both cases. Multiply charged ions can be resolved better than singly charged ones because the resolution increases as the square root of the charge number. The mobility resolution is defined as Rmob=M/ΔM, where ΔM is the width of the ion signal of the mobility M at half maximum, measured in units of the mobility. Since the mobility resolution Rmob depends not only on the diffusion, but also on the finite width of the pulse and on the space charge, for example, it normally has a slightly smaller value than Rd.
Mobility resolutions are generally not very high when compared with mass resolutions in mass spectrometry. Commercial ion mobility spectrometers have resolutions of Rmob=10 to Rmob=40. With a mobility resolution of Rmob=40, two ionic species whose collision cross-sections differ by five percent can be readily separated. Specialized research groups have so far been able to achieve maximum mobility resolutions of Rmob=200, with drift lengths of approximately four to six meters and field strengths of 2000 volts per meter or more, making it possible to differentiate between ionic species whose mobilities differ only by around one percent. Those ion mobility spectrometers whose resolution is above Rmob=60 shall be called “high-resolution” here.
In long drift regions, the ion clouds diffuse very expansively in the radial direction. It has therefore proved expedient to return the ions closer to the axis at certain intervals, every two meters, for example. This can be achieved by ion funnels, which are already known. These ion funnels do not measurably impair the mobility resolution.
It is also possible to keep the ions in the mobility cell on axis by means of RF-generated pseudopotentials. Such an arrangement, installed into a mass spectrometer, was described by A. V. Loboda, U.S. Pat. No. 6,744,043 B2 (2004). The principle of axial focusing of ions by pseudopotentials in a drift region, where the ions are pulled through a damping gas in a DC field, is already disclosed in the patent specification Thomson et al. U.S. Pat. No. 5,847,386 (1998), although there the mechanism was not claimed for the measurement of mobility. The Loboda patent specification, like Thomson et al., proposes an RF ion guide with radial collision focusing for the drift region; the ion guide can be constructed as an RF multipole rod system or as a system of rings.
High-resolution ion mobility spectrometers have the disadvantage of being several meters long. Such a solution is not acceptable for instruments marketed commercially. The research group of David E. Clemmer therefore proposed that the drift region be formed into a closed loop (a type of circular trajectory) with several ion funnels inserted. The ions should enter the circular trajectory via an ion gate, pass through several times and then leave again in a further gate. See also documents U.S. 2010/0193678A1 (D. E. Clemmer et al.), U.S. 2009/0189070 (D. E. Clemmer et al.) and U.S. 2011/0121171A1 (D. E. Clemmer et al.) for this. The research group incorrectly coined the name “Ion Cyclotron Mobility Spectrometry” for this, but the group itself expects relatively major technical problems with this solution. The presence of the gates limits the mobility region because, although it can be extended to a longer drift path and thus higher resolution by means of several orbits of the mobility region, only the region of one single orbit can be measured. The technical design of the gates is difficult if the mobility resolution is to be maintained. A particularly difficult problem which has to be expected, however, is that ions which get onto an outer trajectory by diffusion, and circulate there, will fall behind ions on an inner trajectory due to the longer drift paths and the lower electric field strength. Even if ion funnels are inserted after each quarter of the circular trajectory, the mobility resolution is reduced so much that the value of the proposal must be called into question.
For the construction of compact mobility spectrometers in particular, one therefore has to look for a solution which shortens the overall length, i.e. decreases the “footprint” of the device, but does not diminish the mobility resolution.
We mention only briefly here that for many years arrangements of mobility spectrometers have been known where the isomers are subsequently analyzed with a high-resolution time-of-flight mass spectrometer with orthogonal ion injection, the aim being to obtain mass spectra and mobility spectra of the ion mixtures at the same time. Ion mobility drift cells combined with orthogonally accelerating time-of-flight mass spectrometers have been known from textbooks for forty years.
It is not necessary for the ions to be injected into the drift regions in the form of short ion pulses. The patent application DE 10 2008 025 972.1 (K. Michelmann), equivalent to GB 2 460 341 A or U.S. 2009/0294647 A1, uses an ion mobility spectrometer, for example, which operates with an analog-modulated ion current without ion pulses, the mobility spectrometer being coupled to a mass spectrometer. These arrangements are subject to diffusion broadening of the ultimately obtained mobility spectra in the same way as the ion mobility spectrometers operated with ion current pulses.
Mass spectrometers can only ever determine the ratio of the ion mass to the charge of the ion. In the following, the term “mass of an ion” or “ion mass” always refers to the ratio of the mass m to the number z of elementary charges of the ion, i.e. the charge-related mass m/z. The quality of a mass spectrometer is essentially determined by the mass resolution, amongst other criteria. The mass resolution is defined as Rmass=m/Δm, where Rmass is the resolution, m the mass of an ion, measured in units of the mass scale, and Δm the full width of the mass signal at half maximum, measured in the same units.